1. Field of the Invention
The present invention relates to a printer and a control method thereof and, more particularly, to an improved printer capable of preventing a photosensitive medium from being contaminated due to lack of paper in a consecutive printing process, by using a feed unit having a paper feed distance shorter than an image feed distance, and a control method thereof.
2. Description of the Related Art
Generally, a printer performs a printing process through the processes of charging an image surface of a photosensitive medium such as a photosensitive belt or photosensitive drum with electricity, varying an electric potential level of the image recording surface according to selectively-scanned light, and forming an electrostatic latent image on the image surface. A charged developing solution is added to the electrostatic latent image, to thus visualize an image.
FIG. 1 is a schematic diagram illustrating a general wet printer. Referring to FIG. 1, a photosensitive medium 1 rotates in an endless track under the support of a driving roller 3a, a steering roller 3b and a transfer backup roller 3c. 
A remainder of the electrostatic latent image on the image surface of the photosensitive medium 1 is removed by an eraser 11. Then, the image surface passes through a corona charging device 13, and thus is homogeneously charged to a predetermined potential.
The charged image surface of the photosensitive medium 1 is selectively photosensitized due to a laser beam scanned from a laser scan unit 15, and thus a potential level of the photosensitized region is varied, thereby forming an electrostatic latent image. In addition, a developing unit 16 develops the electrostatic latent image on the image recording surface of the photosensitive medium 1, by using a yellow(Y), magenta(M), cyan(C) or black(K) developing solution from a mixing container 25. That is, the respective color developing solutions are pumped by a pump 26 from the mixing container 25 to a developing roller 17 adjacent to the image surface of the photosensitive medium 1. According to the rotation of the developing roller 17, the developing solutions are added to the electrostatic latent image of the photosensitive medium 1, to form the respective color images. In the developing process, the developing solution dropping into the developing unit 16 is returned to the mixing container 25. The mixing container 25 is filled with the color developing solution where a toner charged to a predetermined potential and a liquid phase carrier are mixed in an appropriate concentration. Here, reference numeral 20 denotes an ink cartridge filled with the high concentration developing solution, and 27 denotes a carrier tank filled with a liquid phase carrier. The developing solution of the appropriate concentration is filled in the mixing container 25 in an appropriate level, by adjusting an amount of the high concentration developing solution and carrier supplied from the ink cartridge 20 and the carrier tank 27, respectively, through valves, V. In addition, the mixing container 25 and the developing unit 16 may be integrated.
After the color image is formed on the image surface according to the developing process, the photosensitive medium 1 is dried in a dry unit 28. Therefore, the liquid phase carrier is removed, while the toner particles remain on the image surface of the photosensitive medium 1. The image of the photosensitive medium 1 is transferred onto a transfer roller 29, and printed on paper 30 passing between the transfer roller 29 and a fixing roller 31.
The paper 30 is picked up from a first feed unit 33 or second feed unit 35 of the printer by feeding rollers 39a, 39b, 39c, and moved along a guide unit 37.
During the consecutive printing process on the paper 30 from the second feed unit 35, the printing process is interrupted if the second feed unit 35 is out of paper 30. Here, since the image to be transferred to the succeeding paper 30 remains on the photosensitive medium 1, it is required to remove the image of the photosensitive medium 1 before re-operating the printer. That is, the residual image is considered as a pollutant. The image remains on the photosensitive medium 1 because a distance between a developing point D of a first color (generally, yellow) and a transfer position B of the image to the paper 30 (hereinafter xe2x80x9cimage feed distance DBxe2x80x9d) is longer than a distance between a paper pickup position C of the feed unit 35 and the transfer position B of the image to the paper 30 (hereinafter xe2x80x9cpaper feed distance CBxe2x80x9d).
Accordingly, when the printer is interrupted, a cleaning process is required to remove the residual image of the photosensitive medium 1. The repeated cleaning processes, however, may shorten the life span of the cleaning unit. In addition, since the photosensitive medium 1 is rotated several times for the cleaning process, the life span of the photosensitive medium 1 is also shortened. Moreover, even when the printer is interrupted, ink is unnecessarily consumed for the image remaining on the photosensitive medium 1.
Accordingly, it is an object of the present invention to provide a printer which can prevent a photosensitive medium from being contaminated due to lack of paper in a consecutive printing process, by using a feed unit having a paper feed distance shorter than an image feed distance, and a control method thereof.
In order to accomplish the above-described object of the present invention, there is provided a printer including: an optical scanner for scanning light to a photosensitive medium according to image information; a developing unit for developing an electrostatic latent image formed by the optical scanner; a transfer unit for transferring the image of the photosensitive medium to paper; an engine driving unit for driving the optical scanner and the developing unit; a feed unit having a paper feed distance shorter than an image feed distance, and being disposed at a predetermined position of a main body, for stacking paper; a residual amount sensing unit for sensing a residual amount of paper; and a control unit for controlling the engine driving unit to perform a normal printing mode when the residual amount of paper exceeds a predetermined limit value, and to perform a contamination prevention mode when the residual amount of paper is below the predetermined limit value.
Preferably, the residual amount sensing unit includes: a paper holding lever securely mounted on one side of the feed unit, for slightly pressing the stacked paper; a paper support unit elastically biased by an elastic member opposite to a direction of gravity; and a residual amount sensor disposed at the feed unit, for sensing a position variation of the paper support unit, and measuring the residual amount of paper.
In accordance with a first embodiment of the present invention, when the residual amount of paper equals the predetermined limit value in the contamination prevention mode, the control unit controls the engine driving unit to interrupt the operation of the optical scanner, and to print all the images of the photosensitive medium on the available paper.
In accordance with a second embodiment of the present invention, when the residual amount of paper equals the predetermined limit value in the contamination prevention mode, the control unit varies a predetermined image pitch, and prints the image of the photosensitive medium on the paper according to such varied image pitch.
In addition, a control method of a printer including an optical scanner for scanning light to a photosensitive medium according to image information, a developing unit for developing an electrostatic latent image formed by the optical scanner, and a transfer unit for transferring the image of the photosensitive medium to paper, includes the steps of sensing a residual amount of paper of a feed unit having a paper feed distance shorter than an image feed distance; comparing the residual amount of paper with a predetermined limit value; performing a normal printing mode and returning to the residual paper amount sensing step when the residual amount of paper exceeds the predetermined limit value; and performing a contamination prevention mode when the residual amount of paper is below the predetermined limit value.
In accordance with the first embodiment of the present invention, the contamination prevention mode step includes the steps of varying an image pitch when the residual amount of paper equals the predetermined limit value; forming an image on the photosensitive medium according to the varied image pitch; printing the image on the paper; re-sensing the residual amount of paper; returning to the image formation step when the residual amount of paper is re-sensed; and temporarily interrupting the printing process when the residual amount of paper is not sensed.
In accordance with the second embodiment of the present invention, the contamination prevention mode step includes the steps of interrupting the operation of the optical scanner by controlling the engine driving unit when the residual amount of paper equals the predetermined limit value; printing all the images of the photosensitive medium on the available paper by driving a corresponding unit when the optical scanning is interrupted; and temporarily interrupting the operation of the printer when the printing step is finished.